The present invention generally relates to combustion devices and more particularly to a method and apparatus for the active control of combustion devices. The proper use of the flame kernel pulse actuator, as disclosed herein, is an improvement over conventional active control techniques used for combustion devices.
Many propulsion systems, such as those used in various tactical missile systems, involve an enclosed combustor. The combustion characteristics of an enclosed combustor, including flammability limits, instability, and efficiency are closely related to the interaction between shear flow dynamics of the fuel and air flow at the inlet and acoustic modes of the combustor. Strong interaction, between the acoustic modes of the combustor and the airflow dynamics may lead to highly unstable combustion. Specifically, unstable combustion may occur when the acoustic modes of the combustor match the instability modes of the airflow. For such conditions, the shedding of the airflow vortices upstream of the combustor tends to excite acoustic resonances in the combustion chamber, which subsequently cause the shedding of more coherent energetic vortices at the resonant frequency. The continued presence of such vortices provides a substantial contribution to the instability of the combustion process.
In the past, passive techniques have been used to control the combustion characteristics. Passive control has historically involved modification to the fuel injection distribution pattern and changes to the combustor geometry. For example, in the dump combustor, nonstandard inlet duct cross-sections were used to control the generation and breakdown of large-scale vortices which play a critical role in driving pressure oscillations and determining the flammability limits. Also, passive control of the combustion characteristics has been achieved by utilizing bluff-body flame holders at the downstream facing step into a dump combustor.
In recent years, active combustion control has received increasing attention. In active control, various control devices such as actuators are used to modify the pressure field in the system and modulate the air or fuel supply to suppress combustion induced pressure oscillations. Typically, a feedback control loop is used to drive the devices using the processed output from a sensor which monitors the flame characteristics or pressure oscillations. Different active control schemes have resulted in suppression of pressure oscillations and extension of flammability limits in a laboratory combustor at ambient pressure and with gaseous fuels.
Several different types of active control devices have previously been used in laboratory experiments. These active control devices include: loudspeakers to modify the pressure field of the system or to obtain gaseous fuel flow modulation; pulsed gas jets aligned across a rearward facing step; adjustable inlets for time-variant change of the inlet area of a combustor; and solenoid-type fuel injectors for controlled unsteady addition of secondary fuel into the main combustion zone. These active control devices have proven to be somewhat successful in suppressing pressure oscillations and extending flammability limits when the combustor operates at low heat release rates and at ambient or low pressures.
Active control has been extended to test conditions which approach operational energy and pressure levels. These tests have pointed towards the need for more effective active control devices which can produce high acoustic power at elevated pressures. However, the existing trend in .active control techniques for a combustor is towards increasing instability and decreasing performance of the combustor with increasing energy and pressure levels.
Consequently, there exists a need for a reliable and relatively inexpensive device which is capable of actively controlling the stability of combustion devices which enhances the overall combustion performance at both high and low operating pressures